The Uncanny X-Men #9 (1965): The X-Men vs The Avengers!

The first of many AvX type events started with Professor X on a solo mission to the center of the Earth to hunt down Lucifer, equipped only with a special wheelchair with tank treads, and quickly escalated to the X-Men fighting The Avengers because the Avengers wanted to help out.

We don’t know this yet, but Lucifer is actually an alien who caused Professor X to lose the ability to walk.